Beyond Supers

As precast concrete products become more and more complex, higher slumps and more vibration are required. Both are expensive and, depending upon the complexity of the element being cast, they may not give the desired results. Here's a project where the vibration was intentionally reduced with excellent results. The key to these successful placements is a new high-range water-reducing admixture for self-consolidating concrete that became available in the U.S. market in late 1998.Experiments with self-consolidating or self-compacting concrete have been reported at technical conferences for many years. The project in this case history is the development of large precast modules for a variety of markets, including hotel rooms and other accommodations. These modules require thinner walls and tighter spacings for weight reduction.In this case, the precast modules have a unique composite wall system using an exterior 2-inch-thick architectural concrete panel, a 2-inch-thick insulation panel and a 1 1/2-inch-thick structural panel. Predictably, the walls contain a lot of reinforcing steel and numerous openings for doors and windows. To make this type of placement possible, A. Rotondo & Sons, Rehoboth, Mass., a subsidiary of Oldcastle Precast, uses a generic polycarboxylate, a new family of high-range water-reducing admixtures. These products offer greater water reduction and greater workability without significantly changing the setting and hardening characteristics of the concrete.The goal is increased workability, with slumps exceeding 12 inches, if that were possible. A field service representative must work with a producer to develop a set of proportions that will work with the user's current materials. So far, the process is a little more trial-and-error than a science, but it is really a matter of fine-tuning rather than radically changing a mixture. The dosage of this admixture is roughly the same as a conventional superplasticizer, and its cost is about twice as much per gallon. So there is an immediate increase in the direct cost of the material. However, a combination of direct and indirect cost savings can offset this increase. This technology does not suit every placement.Sika Corp.'s Mike Campion says the manufacturer plans to begin marketing the self-placing concrete to the ready-mix side of the industry in the near future.